A stable voltage reference immune to temperature and power supply variations is required for high performance analog components produced today. For example, the conversion accuracy of signals from analog to digital and back again in precision coders/decoders (CODECS) is directly dependent on the accuracy of an internal reference. Typically the internal reference is a voltage reference which must be tolerant of power supply voltage variations and noise as well as temperature variations. To keep the cost of CODECS as low as possible, the internal reference should be as physically small as possible and allow for precision trimming of the output voltage, V.sub.BG, if necessary.
A common solution to the internal voltage reference requirement is a circuit known as a bandgap voltage reference. Ideal bandgap voltage references have a predetermined output voltage substantially invariant with variations in temperature by combining the positive temperature coefficient of a thermal voltage (V.sub.T or kxT/q, discussed below), generated by differing voltage drops across forward-biased PN junctions at different current densities, with the negative temperature coefficient of the voltage drop across a forward-biased PN junction (V.sub.BE). In practice, by adding a multiplied thermal voltage, V.sub.T, to V.sub.BE, an output voltage with a predetermined temperature coefficient can be created. Typically, the predetermined temperature coefficient is chosen to be substantially zero.
An exemplary bandgap voltage reference of the prior art is shown in FIG. 2. This simplified bandgap reference 200 is susceptible to power supply voltage (V.sub.DD) variations and noise. A more elaborate variation of this bandgap reference having less susceptibility to power supply voltage variations is discussed in "Analysis and Design of Analog Integrated Circuits", second edition, by P.R. Gray and R.G. Meyer, 1984, pp. 733-737. In FIG. 2, a voltage V.sub.t, which is proportional to V.sub.T, is generated across resistor 201. The temperature coefficient of V.sub.t is also proportional to V.sub.T and is scaled as V.sub.t is scaled to V.sub.T. As will be discussed in more detail in the Detailed Description, below, if each bipolar transistor 203.sub.1 -203.sub.n is substantially identical to bipolar transistor 202 and substantially identical current flows into transistor 202 as into combined transistors 203.sub.1 -203.sub.n, then the current density in transistor 202 is n-times that in each transistor 203.sub.1 -203.sub.n. The differing current density results in a different voltage drop across transistor 202 than across transistors 203.sub.1 -203.sub.n. The difference in voltage drops, designated here as V.sub.t, is forced to appear across resistor 201. To do so, the voltage on the emitter of transistor 202 (node N) must be the same as the voltage on nodeN'. To make the voltage on nodes N and N' the same, fieldeffect transistors (FETs) 204, 205 form a current mirror (not numbered) and FETS 206, 207 form a regulator means (not numbered) which, when coupled to the current mirror, maintains equal output voltages on the source terminals of FETs 206, 207 coupling to nodes N and N', respectively. If FET 206 and FET 207 are the same size with equal current flowing through them from corresponding FETs 204, 205, the gate-to-source voltages of FETs 206, 207 will be the same, resulting in identical voltages on nodes N, N', the outputs of the regulator means. Hence, the difference in the voltage drops across transistors 202 and transistors 203.sub.1 -203.sub.n, V.sub.t, appears across resistor 201.
Since the voltage V.sub.t is dropped across resistor 201, the current through resistor 201, i.sub.t, corresponds to the thermal voltage V.sub.T and has the same temperature coefficient as V.sub.t less the temperature coefficient of resistor 201. More particularly, the temperature coefficient of the current i.sub.t has the temperature coefficient of V.sub.t less the temperature coefficient of resistor 201. In practice, the temperature coefficient of the resistor 201 is much less than the temperature coefficient of V.sub.t. FET 208, responsive to FET 205, mirrors current i.sub.t to produce current I.sub.t. Current I.sub.t passes through resistor 209 to paralleled bipolar transistors 210.sub.1 -210.sub.n, which generate the forward PN junction voltage, V.sub.BE. Transistors 210.sub.1 -210.sub.n correspond to transistors 203.sub.1 -203.sub.n and are substantially the same size. The voltage V.sub.BE adds with the voltage drop across resistor 209 in response to the current I.sub.t, resulting in the bandgap voltage V.sub.BG approximately equaling: EQU V.sub.BG =V.sub.BE +(R.sub.209 /R.sub.201).times.V.sub.T .times.ln(n);
where R.sub.201, R.sub.209 are the resistances of resistors 201, 209, respectively. The where R.sub.201, R.sub.209 pectiThe typcal resistance of resistor 209 is six times that of resistor 201, with resistor 209 consisting of six resistors in series, each resistor having the same resistance as resistor 201.
This bandgap voltage reference 200 suffers from large area requirements due to the 2n+1 bipolar transistors (transistors 203.sub.1 -203.sub.n and 210.sub.1 -210.sub.n and seven resistors (resistors 201 and 209) necessary for the proper generation of the V.sub.t and V.sub.BE voltages. Further, this arrangement suffers from low power supply noise immunity resulting from the FETs 206, 207 of the regulator means having different drain-to-source voltages: the drain voltage of FET 207 is the power supply voltage (V.sub.DD) less a gate-to-source voltage (.apprxeq.1 volt) of FET 205, and the drain voltage of FET 206 is approximately 0.7 volts plus a gate-to-source voltage V.sub.BG (.apprxeq.1 volt). For a five volt power supply, the resulting approximate drain-to-source voltages are 3.3 volts for FET 204, 1 volt for FET 205, 1 volt for FET 206 and 3.3 volts for FET 207. The finite output resistances of the FETs 204, 205, 206, 207 with different drain-to-source voltages cause slight differences in current to flow through FETs 206, 207 and consequently, different voltages on the nodes N, N'. This difference in voltage deviates V.sub.t from the true voltage difference in voltage drops across transistor 202 and transistors 203.sub.1 -203.sub.n. Also, the accurate mirroring of the current i.sub.t by FET 208 is compromised by the finite output resistance thereof. The inaccuracy of current mirroring FETs 206, 207, 208 degrades the performance of the bandgap voltage reference by deviating the output voltage V.sub.BG, and the desired temperature coefficient thereof, from what is desired. The differences in current through FETs 206, 207 and the inaccurate current mirroring by FET 208 are dependent on the power supply voltage V.sub.DD. As disclosed on page 735 of the above-identified reference, the current mirror is compounded and the FETs 206, 207 are made as large as possible to reduce the output resistances thereof in an attempt to reduce the effect the power supply voltage has on the reference. However, different drain-to-source voltages on the FETs remain and, consequently, the immunity of the reference to power supply voltage variations still suffers. Another undesirable result of compounding the current mirrors limits the required power supply voltage (V.sub.DD) to greater than five volts, a common power supply voltage used in integrated circuits.